The invention relates to an amplifier, and, more particularly to an amplifier presented with predistortion to improve linearity of the amplifier.
A known amplifier of cascode, single side, configuration is known from U.S. Pat. No. 5,451,906, and comprises, an emitter common transistor at an input side of the amplifier, coupled to a base common transistor at an output side of the amplifier. Nonlinearity of the amplifier is measured by a decline in the ratio of the output power to the input power, as the frequency range of the RF signal to be amplified by the amplifier. The known amplifier has improved linearity at an upper end of its frequency range, due to placement of an inductor between the transistor at the output side of the amplifier and a feedback circuit. The inductor is coupled to a collector the common base transistor. Return loss stability of the known amplifier is achieved, due to a base bias capacitor at the base of the transistor at the output side of the known amplifier.
According to the invention, linearity of an amplifier over an extended frequency and power range of an RF signal is improved as a result of predistortion at an input side of the amplifier. Nonlinearity of the amplifier is considered to arise from distortion presented by a transistor at an input side of an amplifier. According to an embodiment of the amplifier, predistortion of opposite polarity cancels the distortion presented by the transistor at the input side to improve linearity of the amplifier over an extended frequency and power range.
The predistortion is of opposite polarity relative to distortion presented by the transistor at the input side of an amplifier. The predistortion is presented by a source of small size and low power dissipation, which is suitable for the base bias circuit coupled to the transistor.
For example, the source of predistortion comprises a transistor that is as small as one-sixteenth of the size of the transistor at the input side of the amplifier, which allows the source to be suitable for the base bias circuit of the transistor at the input side of the amplifier.
According to an embodiment of the invention, the source of predistortion is a transistor fabricated together with the first stage transistor and a remainder of the amplifier on a common semiconductor, substrate, for example, semi-insulating GaAs, which enables unitary and consistent circuit construction by MMIC fabrication techniques for control over impedance and phase. According to another embodiment of the invention, the source of predistortion is a diode fabricated together with the first stage transistor and a remainder of the amplifier on a common semiconductor substrate, for example, semi-insulating GaAs, which enables unitary and consistent circuit construction by MMIC fabrication techniques for control over impedance and phase.
According to an embodiment, the transistor at an input side of the amplifier comprises, a first stage transistor at an input side of an amplifier coupled to a remainder of the amplifier. A remainder of the amplifier is considered as a second stage, transistor at an output side of the amplifier.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, according to which: